a) Field of the Invention
The present invention relates to a plasma torch particularly well adapted for the treatment of gases and/or particles and for the deposition of particles onto a substrate.
b) Brief Description of the Prior Art
It has been known for years to use plasma torches for the treatment of gases and/or particles, or for the deposition of particles onto a substrate. Thus, plasma torches can be used for the deposition onto a substrate of one or more layers of particles of different sizes to form, for example, a coating of ceramic particles. Plasma toches can also be used for the treatment of ceramic layers already deposited on a substrate. Plasma torches may further be used for the production or purification of gases and/or particles, in particular ceramic particles, by solid/gas or gas/gas reaction. The particles and/or gases that are injected into the plasma or put in contact therewith, chemically react or are physically modified during their displacement before being deposited onto the substrate, collected in a crucible or recovered.
In most of the plasma torches presently used for the treatment of gases and/or particles or for the deposition of particles onto a substrate, the gases or particles to be treated or deposited are radially injected into the flame. In the case of particles, such an injection calls for a substantial amount of a bearing gas that is usually at ambient temperature, viz. at a temperature substantially different from the temperature of the flame. In practice, this results in a substantial reduction in the temperature of the plasma and consequently in the heat transfer that is required between the plasma and the particles to be deposited, treated or reacted. This also affects the arc voltage. Moreover, radial injection of the particles into the flame causes a segregation of said particles as a function of their sizes, the larger particles having different residence times and heating than the smaller ones. This results in a non-uniform heating of the particles and in poor coatings. In this connection, reference can be made to the preamble of the article of Daniel R. Marantz, entitled "Electromagnetically coalesced multi-arc plasma torch with true axial powder feed" (Thermo Spray Research and Application, Proceedings on the third National Thermo Spray conference, Long Beach, Calif. May 20 to 25, 1990).
To overcome this problem, it has already been suggested to inject the gases and/or particles directly in the middle of the flame. This solution is proposed in the above mentioned article of Daniel R. Marantz, which discloses an apparatus comprising a plurality of plasma torches of conventional structure, which are equally distributed along a circle in such a manner that their flames converge to a same place where the gases or particles to be treated or deposited, are injected.
The main problem with this particular arrangement it that the gases or particles are not injected in the middle of a ring of plasma, but between a plurality of plasma flames whose temperatures, flow rates and viscosities may be different. As a result, the distribution of the injected gases or particles within the flame may be very bad, thereby leading to products or coatings of very bad quality.
In this connection, it is worth mentioning that, due to its flow rate, speed and temperature, the flame of a plasma torch has a given viscosity which can make the injection and the mixing of another gas or particles with the plasma very difficult and/or the distribution of particles within the flame, very irregular.
To tentatively solve this other problem, it has been suggested by another author, Yoshiaki ARATA, in an article entitled "New Technology for processing and evaluating thermo-spray coatings", to use a new kind of plasma torch comprising electrodes that are concentrically mounted to define between them an annular passage which opens into an outlet nozzle. This particular arrangement allows for a much better distribution of gases or particles in the flame, because the gases or particles are injected in the middle of the plasma tunnel that is created when the plasma gas is injected in the annular passage. The gases or particles that are injected in the middle of the plasma tunnel are "sucked in" by the plasma and are thus very well distributed within the flame. However, this arrangement where the central electrode is short and extends upstream of the peripheral electrode, has proved to be a source of problem. Indeed, in such a case, the gases or particles are injected into the arc, thereby interfering with the characteristics of the plasma formed by this arc. It has also been found that the arc, depending on its position, may even "block" the outlet through which the particles are injected into the flame, due to the melting of the coming particles or even the electrode tip.